Predicted anode arc attachment by LTE (local thermodynamic equilibrium) and 2-T (two-temperature) arc models in a cascaded-anode DC plasma spray torch Rodion Zhukovskii, Christophe Chazelas, Vincent Rat, Armelle Vardelle Université de Limoges, IRCER, UMR 7315, 87000 Limoges, France rodion.zhukovskii@unilim.fr Ron Molz Oerlikon Metco (US) Inc. (Westbury, New York, USA) Abstract In DC plasma spray torches, anode erosion is a common concern. It mainly depends on the heat flux brought by the arc and on the dimensions and residence time of the arc attachment to a given location on the anode wall. The latter depend, to a great extent, on the attachment mode of the arc on the anode wall. This paper compares the anode arc attachment modes predicted by a LTE (local thermodynamic equilibrium) and 2- T (two-temperature) arc models that include the electrodes in the computational domain. It deals with a commercial cascaded-anode plasma torch operated at high current (500 A) and low gas flow rate (60 NLPM of argon). It shows that the LTE model predicted a constricted anode arc attachment that moves on the anode ring while the 2-T model predicted a diffuse and steady arc attachment. The comparison between the predicted and measured arc voltage showed that the 2-T prediction is closer to the actual voltage. Also, the post-mortem observation of a new anode ring of the actual plasma torch operated under the same conditions for a short time confirmed a diffuse arc attachment on a new anode. Introduction Anode erosion is a common concern in plasma spraying. It brings about variation in arc dynamics, voltage and attachment mode on the anode wall. It may also modify the development of the arc column inside the plasma torch and the plasma jet issuing from the torch (Ref 1-4); it finally limits the lifetime of the anode and causes production shutdowns and increased operating cost. Therefore, different methods are used by the torch manufacturers to reduce the anode erosion (Ref 5,6). The most common is to limit the residence time of the arc at the same location on the anode wall. An azimuthal displacement of the anode arc attachment is achieved by a swirling injection of the gas. However, the gas swirl tends to progressively decrease along the torch length because of the high viscosity of the hot arc column (Ref 7,8). Thus, a high swirling component at the gas injection is required in order to have a significant effect on the anode arc attachment further downstream. The arc anode attachment fluctuations can also be promoted by the torch design (self-setting arc length torch design) (Ref 1, 9) and/or the operating parameters (e.g.; arc current, nature and gas flow rates of the plasma-forming gas) (Ref 10) or the use of an external axial magnetic field (Ref 11-16). However, if the axial movement of the anode arc attachment occurs over a large portion of the anode, it affects the stability of the plasma jet and so the injection and processing of the powder or suspension in the plasma jet. Common ways to limit the arc movement and get a rather stable plasma jet are either a sudden expansion of the nozzle or an insulating insert between the electrodes (Ref 17). The second method is now common in most of the commercial plasma torches. The nozzle consists, then, in several rings of which the last ring acts as anode. Actually, the erosion of the anode is mainly controlled by the heat flux brought by the arc attachment, which essentially depends on the arc current, nature of the plasma-forming gas and time of residence of the arc attachment in a specific anode area. The anode erosion also depends on the surface area of the arc attachment at the anode wall and therefore on the attachment mode (diffuse, constricted, etc). Cascaded-anode plasma torches are generally operated at lower arc current than conventional torches, the plasma enthalpy increase resulting from an increase in arc voltage (typically around 100-120 V as compared with about 70 V for conventional plasma torch) and thus should benefit of a lower anode erosion, the latter being roughly proportional to the square of arc current. Another approach is to split the arc current in several arcs (Ref 5) either by using a multi-cathode or “by dividing the anode ring into three insulated pie-shaped pieces” (Ref 18). In addition, the nozzle that is traditionally of pure copper because of its high thermal and electrical conductivity, can be protected from erosion by a tungsten liner which has a much higher melting point and heat of fusion than copper (3422 °C and 35.4 kJ·mol -1 , respectively, vs 1085 °C and 13.05 kJ·mol -1 for copper) as it is done in some commercial plasma torches. Controlling the heat flux to the anode and the way it is dissipated in the electrode cooling system should help to increase the lifetime of the anode. A large body of papers deals with the experimental investigation of the heat flux distribution on the anode wall (e.g.; Ref 19-21). Most of the experiments are based on calorimetry methods and yield the total heat flux to anode; they are coupled with other diagnostic methods (e.g.; temperature spectroscopic measurement; Thompson scattering measurement; Langmuir probe; miniature heat conduction probe) or other torch configurations (e.g. split anode) to get an insight into the different contributions of heat flux to anode. However, such measurements are cumbersome and tricky. Therefore, numerical models stand out as the easiest way to Thermal Spray 2021: Proceedings from the International Thermal Spray Conference May 24–28, 2021 F. Azarmi, X. Chen, J. Cizek, C. Cojocaru, B. Jodoin, H. Koivuluoto, Y. Lau, R. Fernandez, O. Ozdemir, H. Salami Jazi, and F. Toma, editors DOI: 10.31399/asm.cp.itsc2021p0360 Copyright © 2021 ASM International® All rights reserved. www.asminternational.org 360 Downloaded from http://dl.asminternational.org/itsc/proceedings-pdf/ITSC 2021/83881/360/487892/itsc2021p0360.pdf by guest on 23 September 2021